1. Field of the Invention
The present invention relates to a method for producing ceramic dielectrics and, more particularly, a method for producing ceramic dielectrics for capacitors or substrates.
2. Description of the Related Art
Recently, as a processing speed of electric equipments becomes faster, the clock frequency or the electric signals of such electric equipments has become more rapid. Therefore, ceramic substrates or ceramic packages applied for such electric equipments have been required to adequately function with such clock frequency or such electric signals. In order to adequately function with such clock frequency or such electric signals, ceramic substrates or ceramic packages need a decoupling capacitor that eliminates switching noise and so on.
A conventional method to provide a decoupling capacitor is to mount the capacitor on a ceramic substrate or a ceramic package. In such method, the ceramic substrate or the ceramic package must provide leads between an electric circuit inside and pads that contact electrodes of the capacitor. It has an advantage that any material or any structure of the capacitor is available. On the other hand, it has a disadvantage that such ceramic substrate or such ceramic package must have a space on it in order to mount the capacitor and further provide complicated leads between the electric circuit inside and pads that contact electrodes of the capacitor.
Another conventional method to provide a decoupling capacitor is to form the capacitor inside the ceramic substrate or ceramic package. The method comprises steps of inserting a material of the capacitor and a conductive paste pattern for electrical connection between green sheets before sintering. This method has great advantages of simplifying the producing process and there is no requirement to have a space on the ceramic substrate or the ceramic package, so that the manufacturing cost can be lower and downsizing can easily be achieved.
As to a basic material for the ceramic capacitor, TiO.sub.2, CaTiO.sub.3, SrTiO.sub.3 are well-known. In recent years, MgTiO.sub.3 --CaTiO.sub.3, Ba(Zn,Nb)O.sub.3 --Ba(Zn,Ta)O.sub.3, BaO--PbO--Nb.sub.2 O.sub.3 --TiO.sub.2, and BaO--Sm.sub.2 O.sub.3 --5TiO.sub.2, having stable property within a wide temperature range, have been known.
As to a conductive paste for an alumina ceramic substrate or alumina ceramic package, a mixture of metal powder such as tungsten or molybdenum and alumina particulates has been used.
A multilayer alumina ceramic has been produced by co-firing the conductive paste and alumina ceramic materials at 1500.degree. C.-1650.degree. C. in a reducing atmosphere. The reducing atmosphere has been applied in order to avoid metal oxidation of conductive paste. However, the reducing atmosphere reacts with the above-mentioned material for the ceramic capacitor, so that such material has changed from insulator to semiconductor. As a result, such changed material leads to an electric short inside the capacitor or change of dielectric property.
On the other hand, it has been known that it is possible to improve the dielectric constant by dispersing a metal powder in a ceramic dielectric. By applying the method to ceramic dielectrics for capacitors or substrates, a capacitor with the dispersed metal powder has been proposed.
The U.S. Pat. No. 5,099,388, which corresponds to Japanese Patent Laid-Open No.91-87091, discloses an alumina multilayer ceramic substrate having a dielectric layer comprising alumina as a base material and 5-50 wt % of one or more elements selected from molybdenum (Mo) and tungsten (W). FIG. 1 is a diagrammatic sectional view of the alumina multilayer substrate shown in the above Japanese Patent, wherein the numeral 12 represents a high dielectric material layer. Inside the alumina multilayer substrate, a high dielectric material layer 12 is formed with a dispersed metal powder such as Mo and metallized layers 13a, 13bare formed on both sides of said high dielectric material layer 12. Said high dielectric material layer 12 and said metallized layers 13a, 13b compose a capacitor. Further, alumina layers 11a, 11b are formed on both sides of said metallized layers 13a, 13b. Said metallized layers 13a, 13b are connected to conductive layers 15a, 15b by conductors 14a, 14b. The above-mentioned structure functions as a decoupling capacitor when electronic devices and the like are mounted, that makes it possible to effectively eliminate electric noise.
Next, a method for manufacturing said capacitor layer is described below. A green sheet for capacitors is formed by the following steps: (a) making a mixture by kneading alumina particulates and one or more elements selected from molybdenum powder, molybdenum trioxide powder, tungsten powder, and tungsten trioxide powder as sintering auxiliaries; (b) forming a predetermined shape from said mixture; (c) sintering said formed mixture in a reductive atmosphere.
In the above-mentioned alumina multilayer ceramic substrate, the relative dielectric constant of the capacitor layer, that is to say, a capacitance of the capacitor layer, is not high enough to function as a decoupling capacitor. In order that the capacitance of the capacitor layer becomes high, it is necessary that said capacitor contains a large amount of metal. Such capacitor, however, easily causes dielectric breakdown.